ABSTRACT
Petroleum is by far the most commonly used source of energy, especially as the source of liquid fuels. In recent years, the average quality of crude oil has become worse. This is reflected in a progressive decrease in API gravity (i.e., increase in density) and a rise in sulfur content (Swain, 1991, 1993, 1997). However, it is now believed that there has been a recent tendency for the quality of crude oil feedstocks to stabilize. Be that as it may, the nature of crude oil refining has been changed considerably.
